Alpha-methylstyrene dimers (AMSDs) may be used as addition fragmentation chain transfer agents in processes of making polymers by free radical polymerization. During polymerization reactions, chain transfer agents may be added to propagating radicals and undergo fragmentation to create new radical forms. AMSDs, unlike some other chain transfer agents, are odorless, easy to handle, and do not cause discoloration or influence the stability of polymers. Additionally, they provide a method of molecular weight control, and AMSD containing functional groups such as hydroxyl, vinyl and amino groups enhance reaction with other functional groups so that polymers formed thereby may be considered telechelic. Telechelic polymers are macromolecules having chains of polymerized monomer comprising reactive functional groups at the terminal ends of the chains. Telechelic polymers are widely used in the synthesis of specialty polymers.
One method of making AMSDs is by the cationic method as described by Savamoto et. al., Macromolecules 14, 467(1981). Though this method provides a fairly inexpensive method of making AMSDs, the method is unable to produced AMSD derivatives (i.e., AMSDs comprising at least one functional group on one or both rings). Alpha methylstyrene monomers including functional groups may be used as starting materials in cationic methods, but functional groups, such as isocyanato and amino groups, are deactivated by the acid used during these methods. In addition, a cationic process tends to provide low yields of AMSDs of which a substantial percentage exits as an "internal" isomer. Please see FIG. 1. Some commercially available preparations of AMSDs believed to be prepared by a cationic method contain about 7% by weight of AMSDs existing as an "internal" isomer. AMSDs existing as an "external" isomer (Please see FIG. 2) show significantly higher reaction rates when used in processes of addition chain transfer compared to "internal" isomers which are relatively inert in such processes.
Another method of making AMSDs is by the radical polymerization process. This process is described in Yamada, et al., Journal of Polymer Science, Part A, Polymer Chemistry, Vol. 32, 2745-2754 (1994), which discloses a method of producing alpha-methylstyrene dimers using benzylbis(dimethylglyoximato)(pyridine)cobalt(III) and a reaction temperature of 60.degree. C. Unfortunately, the method produces low AMSDs yields. Research performed on the polymerization characteristics of alpha-methylstyrene monomers reveals that polymerization occurs until a ceiling temperature, is reached. Then the polymerization of alpha-methylstyrene monomers into polymeric products is inhibited above the ceiling temperature. Martinet et. al., Journal of Applied Polymer Science, Vol. 65, 2297-2313 (1997) reports an alpha-methylstyrene monomer polymerization ceiling temperature of 61.degree. C.